1. Field of the invention
This invention relates to apparatus for precisely positioning an X-Y movable stage with respect to a fixed base and, more particularly, to wafer-stepper apparatus employed in the photolithographic manufacture of monolithic integrated circuits.
2. Description of the Prior Art
In the photolithographic manufacture of monolithic integrated circuits, (1) a wafer comprising a given material, such as silicon or GaAs, is fixed to an X-Y movable wafer stage, and (2) a microscopically-sized configuration of a particular one of a number of stacked layers of an integrated circuit being manufactured is successively photographically imaged and exposed at each of a large plurality of separate specified die sites on the wafer surface, with the wafer being stepped with respect to the image focus from die site to die site by the X-Y movable stage. It is essential that all of the number of stacked layers of a manufactured integrated circuit be precisely in proper alignment with one another at each and every one of the large plurality of separate specified sites on the wafer surface. In order to accomplish this required precision, the displacement of the X-Y movable stage carrying a wafer for the purpose of photolithography is commonly transduced using laser interferometry.
Although it is desirable to move the stage as quickly and precisely as possible during this process to maximize the productivity and yield of the integrated circuits being manufactured, one well known process of achieving this required precise alignment fails to achieve this quick stage movement. In accordance with this well known process, the X-Y stage is commanded to step to a first specified die site and a wafer alignment system is employed to measure the offset between the location of that first die site on the wafer and the focused image produced by the stepper optics. This information is then used to fine position the wafer stage before the exposure. This exact-alignment process is then repeated for each and every one of the large plurality of separate specified die sites on the wafer surface. However, wafer alignment measurements and the consequent repositioning of the wafer stage at every exposure site consumes valuable time that detracts from productivity.
Performing wafer alignment operations on just a few, dispersed die sites on the wafer allows the scale and rotation of the X and Y axes of the entire, previously exposed, layer to be characterized. This information can be used by a wafer stage, that is precise enough, to "blind step" through the rest of the sites without performing the wafer alignment operations at each site. This method of alignment is called Enhanced Global Alignment (EGA). This EGA capability would also be valuable if an alignment target were obscured, but the stepper apparatus could reliably expose such a site anyway.
Laser metering, employing interferometers, is used by a movable wafer stage to measure its spatial position. Such laser metering requires that the wafer stage be equipped with relatively long, narrow plane mirrors. A large source of error that affects the precision of a wafer stage is the straightness or flatness of its plane mirrors. It is not practical to purchase mirrors that have been manufactured to very exacting tolerances. Even if they could be made precise enough, they would be very expensive and, further, small temperature changes and mechanical mounting might cause changes in them after the fact. By mapping the surface of a mirror in situ, all of the aforementioned straightness and flatness errors present can be measured. This permits suitable software algorithms to be used to compensate for such errors.
Current mirror mapping techniques employ a phase measuring interferometer with an expanded wave front to analyze the surface of the mirror. However, these systems are very expensive and longer mirrors would incur extra errors, since, assuming a given limited field of view of a phase-measuring interferometer, the measurements would have to be made by patching together discrete measurements. These systems also require the use and maintenance of reference surfaces. Furthermore, these approaches are more suited for the plane measurement of large area surfaces, as opposed to the linear measurement of a long, narrow mirror surface.
The present invention is directed to a mirror-surface profiler which permits such straightness and/or flatness mirror-surface errors to be quickly and precisely measured at any time.